THERMOPHYSICAL MEASUREMENTS

POSSIBLE HIGH-TEMPERATURE

SUPERCONDUCTIVITY

IN F L U O R I D E M A T E R I A L S Yu. V. Tarbeev, V. V. Kukhar', and M. I. Suvorova

The results are presented of a study of fluorine-containing materials, the chemical compositions of which were chosen from calculations of the valence electron concentration in a single bond. In the resistance--temperature curves of the specimens, a narrow minimum was observed at T = 40~

As is well known, metrology is one field of application of high-temperature superconductivity (HTSC). At the present time, in standards and high-precision measuring methods, superconducting materials with transition temperature lower than 20 K are being used. The most widespread devices, based on the Josephson transitions, have had various technical implementations: SQUIDS, current comparators, and voltage standards. The high cost of the manufacturing technology and servicing of such equipment, and the complexity and large sizes of the devices maintaining the operating temperature of the superconductor (about 4 K) limits their field of application. The requirement of low-temperature superconductors by HTSC materials makes it possible to widen the field of application of the high precision equipment, e.g., in building transportable voltage standards, mobile installations for geology and medicine, etc. At present, some SQUIDS made with HTSC materials have a sensitivity higher than 10-13 T/Hz 1/2 [1]. As shown experimentally, in addition to successful applications in measuring magnetic fields, HTSC SQUIDS can raise the sensitivity of electrical measurements, made at present with semiconductor components [2], by one or two orders. This can be achieved by building superlow-noise operational amplifiers, high-frequency amplifiers, high-sensitivity comparators and transducers. Difficulties in this direction are associated with inadequate manufacturing technology [3] and the necessity to create the associated compounds of the device working at about 80 K. This demands, in particular, the development of new resistive materials with low temperature coefficient of resistance in this temperature range [4]. Without a doubt, wide scientific and technical interest would be shown in a superconducting state near room temperature. An intensive search for new superconductors is being carried out in various laboratories. The main directions of this work are in the choice of doping elements, or complete substitution of individual elements in already known HTSC, and the search for structural analogs of HTSC in new structures. The latter is closely linked with the development of a theory of the superconducting mechanisms, and the role of structural factors in accomplishing these mechanisms, There have been various approaches to this problem. HTSC is associated with the presence in the compound of elements easily changing their degree of oxidation; of special note is the ability of the compound to vary its electrical conductivity over TABLE 1 Crystal lattice type Yetragonal Cubic Rhombohedral Diamond Orthorhombohedral, trigonal Monoc Iinic

Tric] i n i r

Lattice number n

Maximal valency

25 50 75 100

0,1509 0,2134 0,2614 0,3018

I'50 200 250

0,3696 0,42fi8 0,4772

Translated from Izmeritel'naya Tekhnika, No. 6, pp. 43-45, June, 1993.

0543-1972/93/3606-0687512.50 9 1993 Plenum Publishing Corporation

687

TABLE 2 Electron concentration in an atom Ia

Type of crystal lattice (superconductor)

0,0302

0,0427

(Hg)

0,0516

Face-centered cubic

0,0905 0,0954

Electron Electron concen' Iconcentration .tration in one in one bond bond in units of

l,jz 9,6713-10-4 2,7360.10-a 4,8356.10-a

41,359 29,240 24,200

0,04 0,08 0,12 0,36 0,40 0,54

0,0261 0,0306 0,0484

10 13

(PD, NB)

0,1113

13,790 13,060 I'1,215

16

Body-centered cubic

0,1206

10,350

~.64

0,0618

25 50

Tetragonal Cubic (Nb3Sn)

0,1509 0,2134 0,2396

8,272 5,849 5,211

.1,00

0,1209 0,3419 0,4836

Rhombohedra I Diamond

0,261~ 0,3018 0,3696 0,4268

4,776 4,1r 3,377 2,925

0,4505 0,4772

9

63 75 109

150

200 223 250

(YBa2CusOs.r) --

2,52

4,1)0 5,00 6,00

0,6282 0,9071 1,4806 2,0516

2,771

6,70

2,4178

2,616

7,00

2,6760

3 , 0 0 84

me/m ~:' .

(z

-,

ta \ m~

]

0,1.2 1,0

10,0

2

D,K

[25

100

50

Z5

0 i

f

1o

foo

foo0

la f'zrne ~f

iV,,./

Fig. 1 a wide range (from semiconducting type to metallic), by changing the composition within the limits of homogeneity; the stratified character of the materials, perovskite-like theme of the crystal structure [5]. The authors suggest a dependence of HTSC on the electron density in one bond of the crystal lattice. The possibility of estimating this value is offered by structural quantization and a new periodic structure law, establishing the periodicity of structural shape repetitions of chemical elements, which are characterized by the parameter 5 [6]: 688

R,m

R~ m~

/

z"

3

Z.

3 3 2'

3O

4O

4

ZO

50 T,~

30

40

SO 7,~s

Fig. 2 R~m92

e,o

1.5f ~

'

20

25

~

30

tO0

35

40

45

50

r %

Fig. 3

~=0 ,l~aa,,

(1)

where 0.1 is a normalizing multiplier, a is the free structure constant, z~ is a new microstructure constant of the solid phase of the material, and n is the number of the crystal lattice or the quantum structure number, taking integer values from 1 to 250. The parameter 6 is a function of the valence electron density distribution (maximal valence of elements in the group) for interatomic interaction, and characterizes the relative change in interatomic distance in the formation of the solid phase of the material. Mathematically, it is connected with the effective coordination number z by the relationship 1.0030076z = Ap. The 6 values calculated for various types of crystal lattice are given in Table 1. It follows from the table that the valence electron concentration changing the structure by one quantum is 0.04 up to and including the fourth group, and 0.02 above it.

689

o.15 O.tg o, tl~

o~ooje

35

uo Fig. 4

t~5

~0 r.*c

In Table 2 the results are presented of computations from Eq. (1) of valence electron concentrations in various superconductors, and in Fig. 1 is shown a graph of the transkion temperature of known superconductors as a function of the computed electron concentration in one bond, expressed in units of me/mw where m e is the mass of an electron, and m r that of a muon. The graphs obtained form a basis for the proposition that there might exist superconductors with Tc >> 90 K, and Table 2 shows that such superconductors may have double the electron concentration present in already known structures of the type YBa2Cu306. 7 with n = 223. We attempted to check this prediction. For this purpose we chose a material having just the same electron concentration in one bond as in the yttrium ceramic. This, for example, must be possessed by the compound Ca2K4H5.4F13.4. The material was obtained by synthesis (175~ of KHF 2, CaF 2, and HF reagents. By varying the proportions of the reagents, the electron concentration could be varied. The temperature dependence of resistance was studied in specimens which, after pressing (1 ton/cm2) had the form of tablets of diameter 1 cm and thickness 5 mm. The measurements were made with a potentiometric circuit using platinum electrodes. The specimens were dielectrics. The character of their temperature dependence of resistance was determined by the proportions of the reagents and the synthesis time ~-. In Fig. 2a, functions 1-4 correspond to T = 2, 4, 6, and 8 h. In Fig. 2b, the numbers represent: 1) 4 KHF 2 + 1.5 CaF 2 + 2.6 HF; 2) 4 KHF 2 + 2 CaF 2 + 1.4 HF; 3) 4 KHF 2 + 2 CaF 2 + 2 HF2; curves 2' and 2" correspond to two repeat measurements. For the proportions KHF2:CaF2:HF = 4:2:1.4 there was in individual specimens a sharp fall in resistance occurring in a narrow temperature range. The effect was distinguished by its poor reproducibility. To double the electron concentration, we prepared material from a mixture of YBa2Cu306. 7 and Ca2K4Hs.4F13.4 . In Fig. 4 is shown the temperature dependence of resistance of a specimen with 50% YBa2Cu306. 7 + 50% Ca2K4Hs.4F13.4 . For the 50:50 proportions, there was, for all specimens studied, a sharp fall in resistance in the region of 40~ The extent of the fall was 1.5 to 3 orders of magnitude. Leaving the specimen in air, just as repeated measurements, caused a gradual disappearance of the effect. Reports of an anomalous electrical conductivity of fluorine-containing ceramics have appeared in the literature. Often the specimens disclosed a sharp fall in resistance in the region 150-240 K. In [7], the resistance fell to zero with cooling below 155 K, and in [8] to less than 2.10 -7 fl.cm. Although similar phenomena are usually classed as a so-called high-temperature anomaly, they demand an analysis. In the case considered, an explanation of the results is also difficult. The presence of a narrow minimum in the temperature dependence of resistance seems to demonstrate the inclusion, in this temperature range, of an additional conduction mechanism. The effect may be related to the presence of fluoride polymorphism [9], when one of the phases is converted on heating into another modification.

REFERENCES . ,

3. ,

5. 6. 7. 690

V. N. Polushkin, "Bases for the development of high-temperature SQUID systems in an integrated implementation. Part II, High-temperature SQUIDS," Preprint, OIYaI, Dubna (1992), p. 23. V. N. Polushldn, "Electrical SQUIDS with high T c," Proc. ICEC/TCMC, Kiev (1992). V. N. Polushkin, "Bases for the development of high-temperature SQUID systems in an integrated implementation. Part I, Josephson transitions," Preprint, OIYaI, Dubna (1992), p. 2. V. V. Kukhar', S. V. Potapov, and M. 2. Suvorova, Izmereniya, Kontrol', Avtomatizatsiya, 70, No. 2, 51 (1989). I. 1~. Graboi, A. R. Kaul', and Yu. G. Metlin, "The chemistry of solids," Itogi Nauld Tekh., No. 6, 60 (1988). Yu. V. Tarbeev and V. V. Kukhar', Izmer. Telda., No. 7, 3 (1988). S. R. Oyshinsky et al., Phys. Rev. Lett., 58, No. 24, 2579 (1987).

.

9.

Meng Xian-Ren and Ren Yan-Ru, Solid State Commun., 69, No. 3, 325 (1986). A. Wells, Structural Inorganic Chemistry, [Russian translation], Mir, Moscow (1987), po 246.

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